1. Field of the Invention
The present invention relates to passive infrared motion detectors, occupancy sensors and similar devices, and more particularly to the infrared input section of these devices.
2. Description of the Related Art
Passive infrared motion detectors and occupancy sensors employ an array of Fresnel lenses covering an entrance aperture. This lens array is illuminated by thermal infrared radiation from the object of interest. For any particular angle of incidence each of the elements in the array of Fresnel lenses covering the entrance aperture generates a focal spot. The array of Fresnel lenses is designed so that as the object of interest moves across its field of view the system of focal spots moves across the sensitive area of a detector. The varying electrical output signal generated by the detector is processed to yield information about the state of motion of the object of interest.
Each element of the array of Fresnel lenses is designed to focus incident infrared radiation in a small angular range onto the sensitive area of a detector. The angular sectors, in which the elements of the array of Fresnel lenses focus onto one of the active areas of a detector, are interlaced by angular sectors which are not focused onto any sensitive area of any detector by any element of the array of Fresnel lenses. Moving infrared radiators are detected when they move from one angular sector across a boundary into an adjacent angular sector, leading to a rapid change in the amount of infrared power falling on the active area of a detector. Ordinarily all of the sectors are of the same angular size so that the maximum angle through which an object of interest can move without being detected, i.e. the angular resolution of the system, is equal to the angular size of one of these sectors. This assumes that the size and velocity of the radiating object and its distance from the entrance aperture are such that the infrared signal is greater than the minimum that can be detected by the system electronics.
One way to improve the angular resolution of the system is to increase the number of elements in the lens array. More specifically, the angular resolution of the system is approximately inversely proportional to the number of elements in the lens array. Thus, in order to achieve the smallest angular resolution, a lens array with as many elements as possible must be employed. On the other hand, the sensitivity and effective range of the system decrease if the size of the individual lenses of the array is decreased. The phrase xe2x80x9csensitivity of the systemxe2x80x9d refers to the size of the smallest radiating object that can be detected as a function of its distance from the detector. Thus, compromises must be made between the size of the entrance aperture, sensitivity, range and angular resolution of the system. For example, for any desired sensitivity and range there is a minimum size for each of the individual lenses of the array and hence a maximum number of elements for an entrance aperture of fixed size and a corresponding minimum angular resolution. The terms xe2x80x9cfocusxe2x80x9d and xe2x80x9cfocusingxe2x80x9d as used herein are intended to embrace any change in spot size and thus includes partially focusing and defocusing (e.g. dispersing energy).
The present invention is a new input lens configuration which can be employed, for example, to: 1) increase the sensitivity and range of motion detectors and occupancy sensors with an entrance aperture of fixed size without decreasing the angular resolution of the system or, 2) improve the angular resolution of a system with an entrance aperture of fixed size without decreasing the sensitivity or range of the system or, 3) decreasing the size of the entrance aperture required for a given sensitivity, range and angular resolution, or 4) reduce the distance that the unit must protrude in, for example, a wallbox installation in order to achieve acceptable performance at wide angles. In one implementation the angular resolution of the system is reduced to zero, i.e. moving infrared radiators anywhere in the field of view of the system are detected, not just radiators that cross the planes separating a sequence of angular sectors. The relative importance of each of these characteristics of motion detectors and occupancy sensors depends on the application in which the system is employed.
Two-dimensional implementations of the input lens configuration disclosed herein in wallbox installations, for example, have the capability to detect vertical motion as well as horizontal angular motion. Further, such systems can detect horizontal radial motion (e.g. motion directly towards or away from the detector) which is not possible with prior art systems which can only detect infrared radiators moving across the planes which separate a sequence of angular sectors. It is also possible to design two-dimensional systems which can determine the angular size and range of infrared radiators. This is useful in systems which must filter out signals due to various infrared noise sources.
In simplest terms, the infrared input section disclosed herein consists of a lens array, which may be similar to the Fresnel lens array used in the prior art, preceded by one or more, possibly segmented, pre-focusing lenses, which may or may not be Fresnel lenses. For the purpose of illustration, suppose that a certain range and angular resolution can be achieved by employing some particular lens array. If the number of elements of this array is doubled, for example, the angular resolution is improved by approximately a factor of two. However, without changing the size of each element, so as not to affect the sensitivity or range of the system, the size of the array is doubled. This doubling in size can be avoided by employing a pre-focusing lens in front of the customary lens array to focus the beam from any particular incident direction to say, one-half or less of the size of an original lens element. With this configuration the number of elements in the lens array can be effectively doubled, with a corresponding improvement of the angular resolution by a factor of two, without increasing the total size of the lens array or decreasing the sensitivity or range of the system.
In fact, in the above example, both the sensitivity and range of the system are increased as almost all of the infrared power entering the entrance aperture is focused onto the sensitive area of a detector, rather than only the infrared power entering one element of a lens array as in prior art configurations. In other words, in the prior art the infrared power incident on the entrance aperture is focused into many spots, only one of which is effective in activating a detector when the infrared radiator of interest is in a certain angular sector. This is to be contrasted with the input configuration disclosed herein in which there is a single focal spot which contains almost all of the infrared power incident on the entrance aperture. In this situation the amount of infrared power incident on the detector is larger than that incident on the detector in the prior art configurations by a factor approximately equal to the number of elements in the lens array. For some applications the optimum design will employ a small array of pre-focusing lenses as opposed to a single element. It should be noted that depending on the performance characteristics desired, the lens array may be positioned on either side of or in the focal plane of the pre-focusing lens. Further, again depending on the desired performance characteristics, some of the individual elements of the lens array may be converging while others are diverging, neutral or absent.
With a high degree of pre-focusing, the size of the individual lens elements making up the final lens array preceding the detector may become too small to be realized by current Fresnel lens technology. In this situation microlens and diffractive optics technology can be employed to produce elements with the same functionality as an array of Fresnel lenses. These elements can be fabricated of low loss plastic by injection molding with single elements as small as a few infrared wavelengths. The use of current microlens and/or diffractive optics techniques to design and fabricate some, possibly all, of the lens elements will produce more capable systems than those that can be produced with current Fresnel lens technology.
The pre-focusing lens may be curved, flat, or nearly flat and possibly segmented. In general the field of view is limited by Fresnel reflection from the surfaces of the pre-focusing element. This limitation is mitigated by the fact that according to the present invention it is possible to use the entire entrance aperture to collect radiation from one resolution element, as opposed to the prior art in which only a small part of the entrance aperture is used to collect radiation from one angular resolution element. Further, in the present configuration the lens array is enclosed within the unit, i.e., protected, and hence can be made thinner than in the prior art without being subject to accidental damage or casual vandalism. In some applications the optimum design is a hybrid system which employs a traditional array of Fresnel lenses and/or mirrors to cover some angular ranges and the design disclosed herein for the remaining angular ranges.
In general by employing a pre-focusing lens it is possible to achieve the same performance with a much smaller entrance aperture than without a pre-focusing lens. This is of importance, for example, in applications where accidental damage or casual vandalism of the entrance aperture lens/cover is a problem. Depending on the required field of view the pre-focusing lens may be flat or bowed outwards (or inwards) One aesthetically appealing configuration is a rocker switch (e.g. Leviton""s Decora rocker switch) with a small infrared entrance aperture in the center, both vertically and horizontally, of the rocker. Depending on the precise shape of the entrance window, acceptable performance can be achieved with an aperture as small as 4-8 mm horizontally and 10 mm in height. This would convert the traditional rocker switch to an xe2x80x9cautomatic switchxe2x80x9d i.e. an ordinary switch with an occupancy sensor feature. This aesthetically appealing configuration can also be achieved without a pre-focusing lens. However, a pre-focusing lens can be employed to enlarge the field of view and/or decrease the required aperture size for a given range. This technique can be applied to other wiring devices, e.g., toggle switches, dimmers, timers, outlets, etc. These new designs maintain the traditional appearance of the device while adding the occupancy sensor feature in an inconspicuous way. As previously noted in each of these applications a pre-focusing lens may or may not be employed depending on the specified size of the entrance aperture and the required field of view and range.
In general, for any occupancy sensor or motion detector, the field of view can be increased by employing mirrors adjacent to the entrance aperture to reflect wide angle rays towards the center of the system. These mirrors may be positioned before or after the pre-focusing lens or between the lens array and the detector. Further in some applications the optimum system is a hybrid system in which the mirrors direct and/or focus infrared radiation from some angular sectors directly onto a detector, through one lens array to a detector or through both lens arrays to a detector. Infrared radiation from other angular sectors may be processed differently, i.e., by only one or both of the lens arrays.
The optical system disclosed herein can be designed to operate in a number of modes. In the most straightforward design each element of the lens array performs roughly the same function as an element of the Fresnel lens array in the prior art. Specifically the field of view is divided into a number of angular segments. The pre-focusing lens partially focuses infrared radiation within a small range of angles onto one element of the lens array. As the infrared source moves through this angular range the partially focused beam moves across this element of the lens array and the final focal spot moves from some distance off of one side of the sensitive area of a detector to some distance off of the other side of the sensitive area of the detector. If this is repeated for a number of contiguous angular sectors within the field of view of the system the amount of infrared radiation falling on the sensitive area of the detector varies abruptly as the focal spot moves onto or off of the sensitive area of a detector.
In one particularly interesting implementation, the use of a pre-focusing lens leads to qualitative different performance of a motion detector/occupancy sensor than in the prior art. In this implementation the width of the pre-focused beam on the front surface of the lens array is made equal to the width of one element of the lens array. In order to understand the performance of this system, suppose that the infrared source is in a position such that the pre-focused beam just fills one element of the lens array. As the infrared source moves in either direction, the total power illuminating that element of the lens array is reduced and continues to decrease until the beam moves completely off of one side or the other of the element of the lens array. The system can be designed so that, for the entire small range of angles for which the element of the lens array is partially illuminated, this radiation is focused onto the active area of a detector. As the source moves over this small range of angles, the infrared power incident on the detector varies, which produces a corresponding electrical output that is processed to determine the state of motion of the infrared source. This configuration produces a detectable signal at useful source ranges because: 1) of the greater collecting power of the pre-focusing lens, as opposed to the collecting power of a single element of the Fresnel lens array as in the prior art; and 2) the size of each element of the lens array can be greatly reduced, since it is not employed as a collecting element.
If the lens array in the above system is designed so that every other segment of the array is focused on a detector for some small range of angles and these angular ranges are made contiguous, the system behaves in a qualitatively different way than prior art motion detectors/occupancy sensors. Specifically, this system is capable of detecting motion for any angular orientation of the source not only when the source crosses the boundary between an angular sector which illuminates a detector and one which does not. The elements of the lens array which interlace those described above can be simply left unused or employed to focus other, possibly contiguous, angular sectors onto a second detector.
It is not unusual for prior art occupancy sensors and motion detectors to employ a small number of Fresnel lens arrays side by side on the front surface of the unit. These arrays are designed to have different fields of view and/or different ranges. According to the present invention the size of one particular lens element in the array may be made small enough such that many rows of lenses can be employed in a practical system. With such a truly two-dimensional array of lenses, qualitatively different performance can be achieved than in the prior art. Specifically, prior art systems can only detect motion in one angular direction. With a two-dimensional array of lenses motion can be detected in three-directions. For example, with a wallbox or wall mounted system a vertically mounted two-dimensional array can clearly detect vertical as well as angular horizontal motion. Such a system can also detect radial motion in the horizontal plane because an infrared source moving in this direction is also changing its angle with respect to a vertical through the lens array. A properly designed pre-focusing lens and two-dimensional array can also give information about the angular size and range of a moving infrared source. This would greatly increase the noise rejection capabilities of the system.
All of the preceding is equally applicable to, for example, wall and ceiling units, indoor and outdoor units in lighting, heating, ventilation and/or security applications. Also, it is equally applicable to passive and active infrared, optical and microwave systems. Further, the implementations disclosed herein may be used in single technology systems or in combination with motion detectors/occupancy sensors based on other technologies, e.g., active ultrasonic or microwave systems.